I. Field of the Invention:
This invention relates generally to a protection circuit for electrical apparatus and more particularly to a system for detecting when silicon controlled rectifier (SCR) control elements, in solid-state power supply equipment have failed in a shortened circuit.
II. Discussion of the Prior Art:
Since the introduction of the SCR's as high power current switching devices, they have found wide application in solid-state control systems where it is desired to control the current, voltage and/or power supplied to a load. Typically, in single or multi-phase systems, one or more pairs of SCR's may be connected in a back-to-back relationship and in series with a load to which the current, voltage or power is to be controlled and across the AC supply mains. Suitable control means are provided for triggering the SCR's into conduction at an appropiate time in the positive and negative half cycles of the alternating current supply. The time between the zero crossing of the AC supply waveform and the moment that the SCR's are driven into conduction is commonly termed the "firing angle". By providing means whereby the firing angle can be adjusted, the power, current, and/or voltage delivered to the load is controllable.
Various open-loop and closed-loop control devices are known in the art for accurately determining the point in the applied AC waveform where triggering of the SCR's is to occur. In these devices, however, no protection is afforded to the load device in the event that one or more of the SCR's should fail in a shorted condition. When a short-type failure occurs, of course, the current delivered to the load is no longer controlled and is limited solely by the voltage source and the impedance of the load connected to the source. A sustained flow of current to the load through the entire AC cycle can result in damage to the equipment being controlled.
In view of the above, it is advantageous to provide in a switched SCR controller a protective circuit which has the capability of detecting the shortened SCR condition and for disconnecting the load from the supply mains when such a short condition is detected.